Multidimensional visualization of the dynamic evolution of Li metal via in situ/operando methods.

The growing demands for high-energy density electrical energy storage devices stimulate the coupling of conversion-type cathodes and lithium (Li) metal anodes. While promising, the use of these "Li-free" cathodes brings new challenges to the Li anode interface, as Li needs to be dissolved first during cell operation. In this study, we have achieved a direct visualization and comprehensive analysis of the dynamic evolution of the Li interface. The critical metrics of the interfacial resistance, Li growth, and solid electrolyte interface (SEI) distribution during the initial dissolution/deposition processes were systematically investigated by employing multidimensional analysis methods. They include three-electrode impedance tests, in situ atomic force microscopy, scanning electrochemical microscopy, and cryogenic scanning transmission electron microscopy. The high-resolution imaging and real-time observations show that a loose, diffuse, and unevenly distributed SEI is formed during the initial dissolution process. This leads to the dramatically fast growth of Li during the subsequent deposition, deviating from Fick's law, which exacerbates the interfacial impedance. The compactness of the interfacial structure and enrichment of electrolyte species at the surface during the initial deposition play critical roles in the long-term stability of Li anodes, as revealed by operando confocal Raman spectroscopic mapping. Our observations relate to ion transfer, morphological and structural evolution, and Li (de)solvation at Li interfaces, revealing the underlying pathways influenced by the initial dissolution process, which promotes a reconsideration of anode investigations and effective protection strategies.

Seed-Assisted Reversible Dissolution/Deposition of MnO2 for Long-Cyclic and Green Aqueous Zinc-Ion Batteries.

Manganese oxide (MnO2) based aqueous zinc-ion batteries (AZIBs) are considered to be a promising battery for grid-scale energy storage. However, they usually suffer from the great challenge of capacity attenuation due to Mn dissolution and irreversible structural transformation. Herein, full use of the shortcomings is made to design high-performance cathode-free AZIBs. Manganese-based Prussian blue analog (Mn-PBA) is selected as a seed layer to provide a stable MnO2 electrodeposition surface. Thanks to the large specific surface area and manganophilic nature of Mn-PBA, the deposition/dissolution kinetics between Mn2+ and MnO2 are significantly enhanced. Systematic studies revealed the mechanism of MnO2 deposition-dissolution related to the reversible transformation of manganese oxide hydroxide and zinc hydroxide sulfate hydrate. Based on this, the developed cathode-free AZIBs exhibit outstanding rate performance (with a specific capacity of 273.7 mAh g-1 at 1 A g-1) and extraordinary cycle stability (maintaining a specific capacity of 52.3 mAh g-1 after 50 000 cycles at 20 A g-1). Furthermore, the AZIBs with non-toxic, biocompatible materials can be directly discarded after use, without causing pollution to the environment, which is expected to help achieve the sustainable development goals.

Elucidating the Reaction Mechanism of Mn2+ Electrolyte Additives in Aqueous Zinc Batteries.

Aqueous zinc batteries (ZIBs) have attracted considerable attention in recent years because of their high safety and eco-friendly features. Numerous studies have shown that adding Mn2+ salts to ZnSO4 electrolytes enhanced overall energy densities and extended the cycling life of Zn/MnO2 batteries. It is commonly believed that Mn2+ additives in the electrolyte inhibit the dissolution of MnO2 cathode. To better understand the role of Mn2+ electrolyte additives, the ZIB using a Co3 O4 cathode instead of MnO2 in 0.3 m MnSO4 + 3 m ZnSO4 electrolyte is built to avoid interference from MnO2 cathode. As expected, the Zn/Co3 O4 battery exhibits electrochemical characteristics nearly identical to those of Zn/MnO2 batteries. Operando synchrotron X-ray diffraction (XRD), ex situ X-ray absorption spectroscopy (XAS), and electrochemical analyses are carried out to determine the reaction mechanism and pathway. This work demonstrates that the electrochemical reaction occurring at cathode involves a reversible Mn2+ /MnO2 deposition/dissolution process, while a chemical reaction of Zn2+ /Zn4 SO4 (OH)6 ∙5H2 O deposition/dissolution is involved during part of the charge/discharge cycle due to the change in the electrolyte environment. The reversible Zn2+ /Zn4 SO4 (OH)6 ∙5H2 O reaction contributes no capacity and lowers the diffusion kinetics of the Mn2+ /MnO2 reaction, which prevents the operation of ZIBs at high current densities.

Driving reversible redox reactions at solid-liquid interfaces with the electron beam of a transmission electron microscope.

Liquid-cell transmission electron microscopy (LCTEM) has opened up a new way to study chemical reactions at the interface between solids and liquids. However, understanding the effects of the electron beam in the liquid cell has been clearly identified as one of the most important challenges to assess correctly and quantitatively LCTEM data. Here we show that the electron beam can be used to drive reversible deposition/dissolution cycles of copper shells over gold nanoparticles in methanol. Besides revealing the influence of irreversible processes on the kinetic of growth/etching cycles, this study of nanostructure behaviour as a function of the dose rate highlights the possibility to switch the oxidising or reducing nature of liquid environment only with the electron beam. The chemical and electronic processes possibly involved in these tunable redox reactions are qualitatively discussed together with their possible impacts on electrochemical LCTEM experiments.

Observation and quantification of nanoscale processes in lithium batteries by operando electrochemical (S)TEM.

An operando electrochemical stage for the transmission electron microscope has been configured to form a "Li battery" that is used to quantify the electrochemical processes that occur at the anode during charge/discharge cycling. Of particular importance for these observations is the identification of an image contrast reversal that originates from solid Li being less dense than the surrounding liquid electrolyte and electrode surface. This contrast allows Li to be identified from Li-containing compounds that make up the solid-electrolyte interphase (SEI) layer. By correlating images showing the sequence of Li electrodeposition and the evolution of the SEI layer with simultaneously acquired and calibrated cyclic voltammograms, electrodeposition, and electrolyte breakdown processes can be quantified directly on the nanoscale. This approach opens up intriguing new possibilities to rapidly visualize and test the electrochemical performance of a wide range of electrode/electrolyte combinations for next generation battery systems.

Solid Electrolyte Interphase Structure Regulated by Functional Electrolyte Additive for Enhancing Li Metal Anode Performance.

Lithium (Li) metal anodes have become an important component of the next generation of high energy density batteries. However, the Li metal anode still has problems such as Li dendrite growth and unstable solid electrolyte interface layer. Herein, we present a functional electrolyte additive (PANHF) successfully synthesized from acrylonitrile and hexafluorobutyl methacrylate via a polymerization reaction. With extensive analytical characterization, it is found that the PANHF can improve the reversibility and Coulombic efficiency of the Li deposition/dissolution reaction and prevent the growth of Li dendrites by forming a solid electrolyte interphase rich in organic matter on the outer layer and LiF on the inner layer. The results show that the cycling performance of the Li/Li cell was greatly improved in the electrolyte containing 0.5 wt % PANHF. Specifically, the cycling stability of more than 700 cycles was achieved at a current density of 1.0 mA cm-2. Moreover, the Li/NCM811 cell with 0.5 wt % PANHF has a higher capacity of 137.7 mA h g-1 at 1.0 C and a capacity retention of 83.41% after 200 cycles. This work highlights the importance of protecting the Li metal anode with functional bipolymer additives for next-generation Li metal batteries.

Theory-Driven Design of a Cationic Accelerator for High-Performance Electrolytic MnO2 -Zn Batteries.

Aqueous electrolytic MnO2 -Zn batteries are considered as one of the most promising energy-storage devices for their cost effectiveness, high output voltage, and safety, but their electrochemical performance is limited by the sluggish kinetics of cathodic MnO2 /Mn2+ and anodic Zn/Zn2+ reactions. To overcome this critical challenge, herein, a cationic accelerator (CA) strategy is proposed based on the prediction of first-principles calculations. Poly(vinylpyrrolidone) is utilized as a model to testify the rational design of the CA strategy. It manifests that the CA effectively facilitates rapid cations migration in electrolyte and adequate charge transfer at electrode-electrolyte interface, benefiting the deposition/dissolution processes of both Mn2+ and Zn2+ cations to simultaneously improve kinetics of cathodic MnO2 /Mn2+ and anodic Zn/Zn2+ reactions. The resulting MnO2 -Zn battery regulated by CA exhibits large reversible capacities of 455 mAh g-1 and 3.64 mAh cm-2 at 20 C, as well as a long lifespan of 2000 cycles with energy density retention of 90%, achieving one of the best overall performances in the electrolytic MnO2 -Zn batteries. This comprehensive work integrating theoretical prediction with experimental studies provides opportunities to the development of high-performance energy-storage devices.

Balanced Interfacial Ion Concentration and Migration Steric Hindrance Promoting High-Efficiency Deposition/Dissolution Battery Chemistry.

The solid-liquid transition reaction lays the foundation of electrochemical energy storage systems with high capacity, but realizing high efficiency remains a challenge. Herein, in terms of thermodynamics and dynamics, this work demonstrates the significant role of both interfacial H+ concentration and Mn2+ migration steric hindrance for the high-efficiency deposition/dissolution chemistry of zinc-manganese batteries. Specially, the introduction of formate anions can buffer the generated interfacial H+ to stabilize interfacial potential according to the Nernst equation, which stimulates high capacity. Compared with acetate and propionate anions, the formate anion also provides high adsorption density on the cathode surface to shield the electrostatic repulsion due to the small spatial hindrance. Particularly for the solvated Mn2+ , the formate-anion-induced lower energy barrier of the rate-determining step during the step-by-step desolvation process results in lower polarization and higher electrochemical reversibility. In situ tests and theoretical calculations verify that the electrolyte with formate anions achieve a good balance between ion concentration and ion-migration steric hindrance. It exhibits both the high energy density of 531.26 W h kg-1  and long cycle life of more than 300 cycles without obvious decay.

A self-healing neutral aqueous rechargeable Zn/MnO2 battery based on modified carbon nanotubes substrate cathode.

Rechargeable aqueous Zn/MnO2 batteries show great potential for grid-scale storage due to their low cost, high safety, and energy density, yet suffer from continuous capacity decay during operation. Therefore, this work proposes a capacity self-healing aqueous Zn/MnO2 (Zn/cCNTs-MnO2) battery using carboxyl-modified carbon nanotubes (cCNTs) as the cathode substrate, ZnSO4 + MnSO4 mixed aqueous solution as the electrolyte, and Zn foil as the anode. Based on the controllable electrodeposition reaction of MnO2, the specific capacity of Zn/cCNTs-MnO2 batteries can be achieved or recovered by operating several cycles under a low current density (0.1 mA cm-2). Then, the batteries can stably perform under a high current density (1 mA cm-2). By repeating the above steps, a capacity self-healing usage scheme was established, which can significantly improve the cycling performance of Zn/cCNTs-MnO2 batteries. Moreover, the results of the proposed Zn/cCNTs-MnO2 batteries verify the MnO2 electrodeposition mechanism and introduce a novel method for the development of durable aqueous rechargeable Zn/MnO2 batteries.

A High-Energy Aqueous Manganese-Metal Hydride Hybrid Battery.

Aqueous rechargeable batteries show great application prospects in large-scale energy storage because of their reliable safety and low cost. However, a key challenge in developing this battery system lies in its low energy density. Herein, a high-energy manganese-metal hydride (Mn-MH) hybrid battery is reported in which a Mn-based cathode operated by the Mn2+ /MnO2 deposition-dissolution reactions, a hydrogen-storage alloy anode that absorbs and desorbs hydrogen in an alkaline solution, and a proton-exchange membrane separator are employed. Given the benefit derived from the high solubility and high specific capacity of the Lewis acidic MnCl2 in the cathode and the low electrode potential of the MH anode, this aqueous Mn-MH hybrid battery exhibits impressive electrochemical properties with admirable discharge voltage plateaus up to 2.2 V, a competitive energy density of about 240 Wh kg-1 (based on the total mass of the 5.5 m MnCl2 solution and the hydrogen storage alloy electrode system), good cycling stability over 130 cycles, and a desirable rate capability. This work demonstrates a new strategy for achieving high-performance and low-cost aqueous rechargeable batteries.